Light waves may be made to carry information by modulating a light source, often a laser source, to change various properties of the light, such as its amplitude, phase, frequency, wavelength, etc. The light waves may be in the visible spectral band, the infrared spectral band, or another region of the electromagnetic spectrum. In some cases, an underlying signal, such as a radio frequency signal, may be modulated via amplitude, phase, or frequency modulation, or any combination thereof, and the light source may be modulated by the underlying signal. Optical receivers receive the light waves and measure properties or variations of the light wave, such as the amplitude, phase transitions, and the like, from which the underlying signal and the information may be recovered.
Quadrature Amplitude Modulation (QAM) of light signals includes both phase modulation and amplitude modulation. Information encoded in phase and amplitude modulation may include transmitted communication data, or may include other information such as information about the source of the optical signal, interaction of the optical signal with an object, the optical channel through which the optical signal traveled, and/or objects with which it interacted. Compared to typical amplitude modulation receivers, receivers capable of demodulating phase modulation can be significantly more complex, requiring precision optics, local oscillators, gratings (such as Fiber Bragg Gratings), and/or delay line interferometers (DLI), etc. Receivers for high-order coherently encoded signals require frequency controlled lasers to serve as local oscillators (LO), multiple detectors, and significant digital signal processing (DSP).